9,541 research outputs found
Angular spectrum of quantized light beams
We introduce a generalized angular spectrum representation for quantized
light beams. By using our formalism, we are able to derive simple expressions
for the electromagnetic vector potential operator in the case of: {a)}
time-independent paraxial fields, {b)} time-dependent paraxial fields, and {c)}
non-paraxial fields. For the first case, the well known paraxial results are
fully recovered.Comment: 3 pages, no figure
Ideal Linear Chain Polymers with Fixed Angular Momentum
The statistical mechanics of a linear non-interacting polymer chain with a
large number of monomers is considered with fixed angular momentum. The radius
of gyration for a linear polymer is derived exactly by functional integration.
This result is then compared to simulations done with a large number of
non-interacting rigid links at fixed angular momentum. The simulation agrees
with the theory up to finite size corrections. The simulations are also used to
investigate the anisotropic nature of a spinning polymer. We find universal
scaling of the polymer size along the direction of the angular momentum, as a
function of rescaled angular momentum.Comment: 7 pages, 3 figure
Spin Precession and Avalanches
In many magnetic materials, spin dynamics at short times are dominated by
precessional motion as damping is relatively small. In the limit of no damping
and no thermal noise, we show that for a large enough initial instability, an
avalanche can transition to an ergodic phase where the state is equivalent to
one at finite temperature, often above that for ferromagnetic ordering. This
dynamical nucleation phenomenon is analyzed theoretically. For small finite
damping the high temperature growth front becomes spread out over a large
region. The implications for real materials are discussed.Comment: 4 pages 2 figure
Collision of Polymers in a Vacuum
In a number of experimental situations, single polymer molecules can be
suspended in a vacuum. Here collisions between such molecules are considered.
The limit of high collision velocity is investigated numerically for a variety
of conditions. The distribution of contact times, scattering angles, and final
velocities are analyzed. In this limit, self avoiding chains are found to
become highly stretched as they collide with each other, and have a
distribution of scattering times that depends on the scattering angle. The
velocity of the molecules after the collisions is similar to predictions of a
model assuming thermal equilibration of molecules during the collision. The
most important difference is a significant subset of molecules that
inelastically scatter but do not substantially change direction.Comment: 7 pages, 6 figure
Structure of strongly coupled, multi-component plasmas
We investigate the short-range structure in strongly coupled fluidlike plasmas using the hypernetted chain approach generalized to multicomponent systems. Good agreement with numerical simulations validates this method for the parameters considered. We found a strong mutual impact on the spatial arrangement for systems with multiple ion species which is most clearly pronounced in the static structure factor. Quantum pseudopotentials were used to mimic diffraction and exchange effects in dense electron-ion systems. We demonstrate that the different kinds of pseudopotentials proposed lead to large differences in both the pair distributions and structure factors. Large discrepancies were also found in the predicted ion feature of the x-ray scattering signal, illustrating the need for comparison with full quantum calculations or experimental verification
Internal dissipation of a polymer
The dynamics of flexible polymer molecules are often assumed to be governed
by hydrodynamics of the solvent. However there is considerable evidence that
internal dissipation of a polymer contributes as well. Here we investigate the
dynamics of a single chain in the absence of solvent to characterize the nature
of this internal friction. We model the chains as freely hinged but with
localized bond angles and 3-fold symmetric dihedral angles. We show that the
damping is close but not identical to Kelvin damping, which depends on the
first temporal and second spatial derivative of monomer position. With no
internal potential between monomers, the magnitude of the damping is small for
long wavelengths and weakly damped oscillatory time dependent behavior is seen
for a large range of spatial modes. When the size of the internal potential is
increased, such oscillations persist, but the damping becomes larger. However
underdamped motion is present even with quite strong dihedral barriers for long
enough wavelengths.Comment: 6 pages, 8 figure
Ising Dynamics with Damping
We show for the Ising model that is possible construct a discrete time
stochastic model analogous to the Langevin equation that incorporates an
arbitrary amount of damping. It is shown to give the correct equilibrium
statistics and is then used to investigate nonequilibrium phenomena, in
particular, magnetic avalanches. The value of damping can greatly alter the
shape of hysteresis loops, and for small damping and high disorder, the
morphology of large avalanches can be drastically effected. Small damping also
alters the size distribution of avalanches at criticality.Comment: 8 pages, 8 figures, 2 colum
A Search for the Optical Counterpart of the Luminous X-ray Source in NGC 6652
We examine images of the field of X1832-330, the luminous (Lx ~ 10^36 erg/s)
X-ray burst source near the center of the globular cluster NGC 6652, in order
to identify the optical counterpart for further study. U and B ground-based
images allow us to set a limit M_B > 3.5 for the counterpart at the time of
those observations, provided that the color is (U-B)_0 ~ -1, similar to the
sources known in other clusters. Archival Hubble Space Telescope observations
survey most but not all of the 1 sigma X-ray error circle, and allow us to set
limits M_B > 5.9 and M_B > 5.2 in the WF/PC and WFPC2 regions, respectively. In
the WF/PC images we do weakly detect a faint object with UV-excess, but it is
located 11.7'' from the ROSAT X-ray position. This considerable (2.3 sigma)
discrepancy in position suggests that this candidate be treated with caution,
but it remains the only reasonable one advanced thus far. We measure for this
star m_439 = 20.2 +- 0.2, (m_336 - m_439) = -0.5 +- 0.2, and estimate M_B =
5.5, (U-B)_0 = -0.9, similar to other known optical counterparts. If this
candidate is not the identification, our limits imply that the true
counterpart, not yet identified, is probably the optically-faintest cluster
source yet known, or alternatively that it did not show significant UV excess
at the time of these observations. Finally, we assess the outlook for the
identification of the remaining luminous globular cluster X-ray sources.Comment: 15 pages including 5 figures and no tables. Accepted for publication
in The Astronomical Journal; to appear in Volume 116, September 1998. A
preprint with full resolution figures may be downloaded from
http://www.astro.washington.edu/deutsch/pubs
Pattern formation in quantum Turing machines
We investigate the iteration of a sequence of local and pair unitary
transformations, which can be interpreted to result from a Turing-head
(pseudo-spin ) rotating along a closed Turing-tape ( additional
pseudo-spins). The dynamical evolution of the Bloch-vector of , which can be
decomposed into primitive pure state Turing-head trajectories, gives
rise to fascinating geometrical patterns reflecting the entanglement between
head and tape. These machines thus provide intuitive examples for quantum
parallelism and, at the same time, means for local testing of quantum network
dynamics.Comment: Accepted for publication in Phys.Rev.A, 3 figures, REVTEX fil
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